1. Field of the Invention
The present invention relates to a direct-write method and apparatus for the selective separation of one layer of material from another layer using an ultrashort pulse source of electromagnetic radiation.
2. Description of the Related Art
U.S. Pat. No. 7,528,342 by Dashi incorporated herein by reference in its entirety describes a method and apparatus for selective material removal of at least one layer of material on another using an ultrashort pulse laser. In this patent Dashi states “In ultrafast laser processing the threshold fluence of the material is clearly defined. Hence by controlling the pulsed laser fluence, material with lower threshold fluence can be selectively removed without ablating the underlying material of higher threshold fluence.” Further along in the text Deshi states “It is not imperative that the entire overlying layer have a lower threshold fluence then the underlying layer (which should not be ablated). For precise machining, only the layer immediately above the underlying layer where the ablation/machining should stop, need to have the threshold fluence lowers (sic) than the underlying layer.”
Thus Dashi relies on the well-known fact that a lower ablation threshold material can be removed from a higher ablation threshold material by precisely controlling the fluence of the light incident on the lower ablation material so that it ablates the lower ablation threshold material but not the higher ablation threshold material.
U.S. Pat. No. 6,333,485 by Haight, et al included herein in its entirety by reference produces a similar outcome as it is applied to the repair of an opaque defect on a photomask without damaging the underlying layer.
U.S. Pat. No. 6,159,832 by Mayer, included herein in its entirety by reference describes the precision deposition of electrically conducting film by the forward transfer of a metal onto a substrate.
U.S. Pat. No. 6,815,015 by Young incorporated herein in its entirety by reference, describes the advantages of direct-write and the forward transfer of a rheological fluid.
United States Patent Application 20080139075 by Birrell, et al incorporated herein in its entirety by reference describes deposition repair apparatus and methods that employ methods similar to U.S. Pat. Nos. 6,825,015 and 6,159,832.
These patents and patent applications rely on the precise, well defined difference in ablation threshold of the two layers to achieve to their objective. More specifically, they rely on the fact that the layer that is ablated has a threshold for ablation that is lower than that of the underlying substrate. And thus by arrange the incident fluence to be above the threshold for ablation of the layer removed and below the threshold for ablation of the underlying layer or substrate, it is possible to remove or separate the lower ablation threshold from the higher threshold ablation layer without damaging the underlying layer or substrate with the higher ablation threshold.
This condition misses an important understanding that is enabling in several identifiable situations and is the subject of this invention. When the threshold for ablation at the interface between two materials is lower than the threshold for ablation of the layers that form the interface, it is possible to induce the separation of one layer from another without expending as much energy as is needed when ablating the bulk. And thus by exceeding the threshold for ablation at the interface with an ultrashort pulse of electromagnetic radiation it is possible to achieve separation between two layers of material under conditions that imbue the process, the product, and consequently the end use device, with all the well-known benefits that accrue to the use of ultrashort pulses of electromagnetic radiation to separate layers of materials; benefits such as reduced heat affected zone (HAZ) reduced or eliminated recast layer, reduced splattering, undesirable delamination of adjacent layers or structures, the creation of microcracks, a well-defined and highly repeatable zone of separation of the two materials, and the highly deterministic precision and reproducability of the separation process.
An important additional benefit is the ability to create a zone of separation whose size is smaller, sometimes even substantially smaller, than the size of the beam incident on the interface. Thus, for example, in some situations it would be advantageous to use ablation at an interface to induce the forward transfer a layer of material of sub-micron dimensions to another surface by precisely controlling the fluence of the incident pulse such that only the very top of the spatial profile of the beam is above the threshold for ablation at the interface.
In some cases it is possible, and even desirable, to induce this separation by locally exceeding this threshold for ablation at the interface as, for example, when it is desirable to increase the rate of separation so that the one layer can be patterned more rapidly and thus with higher throughput and lower cost.
In addition, there are cases wherein it is possible to remove the higher ablation threshold material from the lower ablation threshold layer without damaging the lower ablation threshold layer or the material or structures adjacent to it, as is the case, for example, when it is desirable to use ultrashort pulses of radiation to ablated a thick layer of material with a higher ablation threshold from a lower ablation threshold polymer substrate. In this instance the higher ablation threshold layer of material may be first thinned to dimensions wherein an evanescent wave of sufficient energy to exceed the threshold for ablation at the interface builds up in the interface and causes the two layers of material to separate.
In paper M503 presented at the ICALEO Conference in 2006 titled “NANOSCALE ANALYSIS OF LASER ABLATED THIN FILMS USE IN INDUSTRIAL MANUFACTURING OF FLAT PANEL DISPLAYS” by Matt Henry, et al, incorporated herein by reference in its entirety, the authors briefly comment on several sources that might be used to pattern flat panel displays. With respect to using an ultrashort pulse of femtosecond or picosecond duration. The authors write state “ . . . in all cases ultrafast lasers have relatively low pulse energies—in the order of 1 mJ. Thus to achieve thin film removal they are focused to fine spot sizes in the order of 10 μm to achieve sufficient energy density (Fluence). This makes them unsuitable for creating large area TCO structures at the commercial rates required for large area FPD manufacture, although smaller scale FPD applications such as OLED may be viable.” Contrary to this statement this innovation clearly demonstrates that it is possible to create large area TCO structures at commercial rates in TCO films in the manufacture of devices such as touch screens and FPDs with pulse energy substantially below 1 mJ/pulse.
U.S. Pat. No. 5,652,083 by Kumar, et al, incorporated herein in its entirety by reference, describes a method for fabricating a display cathode by “ . . . patterning and etching . . . ” Clearly this patent refers to a chemical (wet etching) process. No mention is made of using an ultrashort pulse source that would imbue the method with all the benefits in quality that accrue to the use of this technology without having to employ wet etching process.
United States Patent Application 20050074974 incorporated herein in its entirety by reference, Stoltz describes “ . . . methods and systems for ablation based material removal configuration for use in semiconductor manufacturing that includes the steps of generating an initial wavelength-swept-with-ti-me (sic) optical pulse in an optical pulse generator, amplifying the initial pulse, compressing the amplified pulse to a duration of less than about 10 picoseconds and applying the compressed optical pulse to the wafer surface, to remove material from, e.g., wafer surface.” A “ . . . wavelength-swept-in-time . . . ” source is another way of describing a chirped pulse amplifier system known to those skilled in the art of laser source technology whose invention is attributed to Donna Strickland and Gerard Mourou, Optics Communications, Volume 56, Issue 3, 1 Dec. 1985, Pages 219-221. This application focuses on semiconductor materials and makes no mention of TCO on transparent substrates, nor does it make use of the fact that the threshold for ablation is lower at an interface compared to the threshold for ablation of the bulk material.
United States Patent Application 20050226287 incorporated herein in its entirety by reference, reads in part “Often, portions of such layers must be removed and/or inspected, while causing minimal damage to the underlying substrate. In such cases, the optimal choice of laser wavelength often depends upon the nature of the substrate material. In the case of a thin target layer (or layers) on a transparent substrate, near IR femtosecond pulses may be preferred since they can be precisely focused upon the target layer without interacting with the transparent substrate (whereas linear absorption might be significant when using a UV source). In the case of a thin transparent layer (or layers) on an opaque substrate, UV femtosecond pulses may be preferred because of their high absorption coefficient (and correspondingly thin optical penetration depth) thereby confining energy deposition to a thin layer at the surface.” Again, this application relies on the threshold of ablation at the surface rather than at the interface between two surfaces.
As noted in United States Patent Applications 20090107707 and 20090107707, a conductive paste containing mainly metal powder and is widely used in electronic device components since it shows excellent conductive properties and for example, is used as material for an electrically-conductive path when forming an electric circuit on a wiring board, display or touch screen. The conductive paste is produced in the form of a paste by dispersing metal powder and glass frit in an organic vehicle. This conductive paste is applied to a ceramic, glass substrate or the like by screen printing or the like so as to form a wiring pattern. When the conductive paste is sintered at high temperatures, the organic vehicle evaporates and the metal powder is sintered so as to form a continuous film. Such conductive pastes provide excellent conductive properties since the metal powder is sintered to form a continuous solid film.
United States Patent Application 20080128397 incorporated herein in its entirety by reference makes no mention of employing the lower threshold for ablation at the interface or the evanescent wave propagating into the interface to separate one layer of material from another.
Kasuga, et al in U.S. Pat. No. 7,198,736 writes “A conductive silver paste including silver powders is printed on a surface of a base material using various types of printing methods, or is applied thereon using various types of coating methods, dried, and is further heat-treated as required, thereby forming a conductive film such as a conductor wiring,” and “As various types of electronic equipment are miniaturized, it is also required that the conductor wiring composed of the conductive film is made fine. For example, in a conductor wiring having a line shape, it is required that the line width of the conductor wiring and the space width between adjacent conductor wirings are respectively not more than 100 microns. In the future, it is predicted that the conductor wiring is required to be made finer,” and “When the above-mentioned conductor wiring is formed by a screen printing method using the conductive silver paste, for example, a screen having a screen opening sufficiently smaller than the line width of the conductor wiring and the space width between the conductor wirings must be used in order to satisfactorily reproduce the fine plane shape. However, such a screen having a small opening is liable to be clogged with large-diameter silver powders having an average particle diameter of not less than 1 micron. When the screen is clogged, there occurs such inferior printing that the printed conductor wiring is scratchy and a line of the conductor wiring is broken halfway. Particularly in an edge portion of the conductor wiring, the granularity of sufficiently larger silver powders than the fine plane shape is noticeable, so that there also occurs such inferior printing that it is recognized that the edge portion of the conductor wiring is blurred.”
Thus the drive toward smaller electronic components encounters a problem when attempting to screen print conductive paste material on a substrate to create conductive features such as wires with at least one dimension below about 100 microns. Kasuga, et al above referenced patent deals with using silver conductive paste composed of particles of smaller size. Here we apply this invention to shape sintered conductive pastes using standard production techniques and direct-write the pattern therein that in order to create features such as wires on sub-100 micron dimensions.
Medical implants are another area where the invention described herein will be useful. Here there is a continuous drive to fabricate devices such as electrode arrays that are smaller, with improved functionality and at the same time are robust enough to tolerate the stresses that are incurred during implantation. As an example, the electrode array of the cochlear implant consists of a bundle of platinum wires assembled by hand under a microscope. This fabrication induced stress compromises the plasticity of the platinum electrodes which can result in electrical breaks during the additional stress caused by surgical implantation in the scala tampani. Similarly electrode arrays that are implanted in the brain for use in the local detection and monitoring of brain waves, or for providing a signal to actuate or control a bionic device, would benefit from a precise and reliable method of fabrication that have minimal impact on brain tissue during implantation.